Microfiber and nanofiber membranes, or “felts”, have a variety of different uses for both biological and industrial applications. For example, felts are useful in providing for textile reinforcement, protective clothing, catalytic media, agricultural applications, sensors for environmental, medical, and military monitoring, biomedical applications (e.g., bioseparations, tissue engineering and wound dressings), electronic applications (e.g., capacitors, transistors and diodes), and space applications (e.g., solar sails and backing structure for space mirrors). Microfiber and nanofiber felts are particularly well suited for purifying biological substances, such as proteins, nucleic acids, carbohydrates, bacteria, viruses, cells, and the like. They are useful in all fluid applications, both liquid and gaseous.
The biopharmaceutical therapeutics industry is expanding as more and more biopharmaceuticals are approved for sale. In addition, biologically based diagnostic tools are widely used to perform high throughput, sensitive diagnostic testing of various disease states. For both therapeutics and diagnostics, biological substances (e.g., recombinant proteins, monoclonal antibodies, viral vaccines, and nucleic acids) must be efficiently produced and purified for use.
Conventional purification methodologies include the separation of desired biological substances from byproducts and other contaminants using, for example, packed microbeads for adsorption/chromatography, ultrafiltration, and precipitation/crystallization. These conventional separation methods provide adequate results for many biological applications, but are limited in terms of yield, processing time and degree of purity. These limitations are primarily due to slow diffusion rates of relatively large biomolecules, which limits the ability of the substance being purified (i.e., the “target substance”) to access available binding sites deep within the separation matrix. In addition, these systems can only be used for a limited number of cycles, and some can only be used once.
Ion-exchange (IE) and hydrophobic interaction (HI) adsorption/chromatography are two examples of more robust conventional separation technologies that are widely used for separation of biological substances. They are generally less efficient overall than separation technologies based on specific affinity, such as antibody-based separations, but if separation conditions are carefully selected, they are still useful for purifying many target substances from undesirable byproducts and impurities.
While affinity-based adsorption/chromatography may be more efficient than IE and HI, it is generally more difficult and expensive to manufacture, because of the complexity of producing and purifying biological ligands, such as monoclonal antibodies and nucleic acids. Such ligands are also often very sensitive to environmental conditions (e.g., temperature, pH, ionic strength, etc.) and can easily become deteriorated such that the affinity interaction required for adsorption is destroyed. In addition, the binding interaction is sometimes difficult to disrupt without harsh conditions that may lessen the biological activity and hence the usefulness of the target substance and/or the reusability of the purification media.
Membranes that are useful for purification of biological substances have been described. (See, e.g., Bioprocessing for Value-Added Products from Renewable Resources, Shang-Tian Yang, Ed., Chapter 7.) Recently, membrane adsorption/chromatography using nanometer diameter fibers constructed into mats of controlled thickness (i.e., “nanofiber felts”) has shown great promise for use in bioseparations (Todd J. Menkhaus, et al., “Chapter 3: Applications of Electrospun Nanofiber Membranes for Bioseparations”, in Handbook of Membrane Research, Stephan V. Gorley, Ed.) Such nanofiber felts are superior to microfiber felts, because pore sizes, affinity characteristics, as well as other performance criteria, can be more precisely controlled.
While previously described single component nanofiber felts have provided promising results, they are often less efficient than would be desirable in terms of stability of the felts, as well as material and time requirements. This is particularly true when the target substance is only present in the starting material to be purified at a low concentration, and contaminants and/or the byproducts of synthesis are abundant. Thus, there exists a need to improve the stability of the felts and the purification efficiency of biological products. The embodiments disclosed below satisfy that need.